Epidemiological studies have shown a strong relationship between the incidence of suicides, affective episodes and the seasons. In order to understand this association, we have studied patients who regularly become depressed at a certain time of year. Most of these patients become depressed in winter and recover or become hypomanic in spring or summer. We have studied over 100 such patients with Seasonal Affective Disorder (SAD). Most are women with an onset of illness in their twenties. During depressions they become lethargic, overeat, oversleep, gain weight and withdraw from friends and family. Although they rarely require hospitalization and hold their jobs, their level of functioning deteriorates. Most SAD patients note that their depressions improve when they travel south in the winter. Although the above pattern of SAD is the one most frequently encountered, an analysis of 602 questionnaires revealed that other patterns, including regular summer and spring depressions, also occur. Children as well as adults may be affected by SAD and when this occurs, the symptoms may be somewhat different than in the case of adults. Anecdotal reports suggest that after winter light treatment, the course of the condition during subsequent seasons may be altered. Ongoing studies involve: (1) systematic follow-up of patients who have been treated with light therapy for their winter depressions; (2) studies of children and adolescents with SAD; (3) further delineation of the relationship between climatic variables and mood, sleep and behavior.